Generally the higher your upload speed, the smoother the measurements. Some devices and browsers are better or worse than others at displaying a smooth upload speed.Buffer Bloat can also make for a bumpy upload speed.I hope this info helps you before wasting hours of troubleshooting!
If you repeatably get a lower result look more carefully at your browser and your PC. Install a clean, current, browser (Firefox or Chrome) and try it with that before concluding anything.
Because browsers typically download files and store them on disk the upper limit of this speed test is going to be limited by the fastest write speed of your PC disk drive.
In the case of a spinning disk this might be as low as 500 megabit.
By using Multiple IIS sites from one server you can also gain very granular control as that what is enabled or disabled.
This would make more sense to me if the updates were installed a day (or week) before the error occurred.
By far the worst thing about Sophos 8 was the EM Library.